It 's official : U.S. President Barack Obama wants lawmakers to weigh in on whether to use military force in Syria .

Obama sent a letter to the heads of the House and Senate on Saturday night , hours after announcing that he believes military action against Syrian targets is the right step to take over the alleged use of chemical weapons .

The proposed legislation from Obama asks Congress to approve the use of military force `` to deter , disrupt , prevent and degrade the potential for future uses of chemical weapons or other weapons of mass destruction . ''

It 's a step that is set to turn an international crisis into a fierce domestic political battle .

There are key questions looming over the debate : What did U.N. weapons inspectors find in Syria ? What happens if Congress votes no ? And how will the Syrian government react ?

In a televised address from the White House Rose Garden earlier Saturday , the president said he would take his case to Congress , not because he has to -- but because he wants to .

`` While I believe I have the authority to carry out this military action without specific congressional authorization , I know that the country will be stronger if we take this course , and our actions will be even more effective , '' he said . `` We should have this debate , because the issues are too big for business as usual . ''

Obama said top congressional leaders had agreed to schedule a debate when the body returns to Washington on September 9 . The Senate Foreign Relations Committee will hold a hearing over the matter on Tuesday , Sen. Robert Menendez said .

Transcript : Read Obama 's full remarks

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U.N. inspectors leave Syria

Obama 's remarks came shortly after U.N. inspectors left Syria , carrying evidence that will determine whether chemical weapons were used in an attack early last week in a Damascus suburb .

`` The aim of the game here , the mandate , is very clear -- and that is to ascertain whether chemical weapons were used -- and not by whom , '' U.N. spokesman Martin Nesirky told reporters on Saturday .

But who used the weapons in the reported toxic gas attack in a Damascus suburb on August 21 has been a key point of global debate over the Syrian crisis .

Top U.S. officials have said there 's no doubt that the Syrian government was behind it , while Syrian officials have denied responsibility and blamed jihadists fighting with the rebels .

British and U.S. intelligence reports say the attack involved chemical weapons , but U.N. officials have stressed the importance of waiting for an official report from inspectors .

The inspectors will share their findings with U.N. Secretary-General Ban Ki-moon Ban , who has said he wants to wait until the U.N. team 's final report is completed before presenting it to the U.N. Security Council .

The Organization for the Prohibition of Chemical Weapons , which nine of the inspectors belong to , said Saturday that it could take up to three weeks to analyze the evidence they collected .

`` It needs time to be able to analyze the information and the samples , '' Nesirky said .

He noted that Ban has repeatedly said there is no alternative to a political solution to the crisis in Syria , and that `` a military solution is not an option . ''

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Obama : ` This menace must be confronted '

Obama 's senior advisers have debated the next steps to take , and the president 's comments Saturday came amid mounting political pressure over the situation in Syria . Some U.S. lawmakers have called for immediate action while others warn of stepping into what could become a quagmire .

Some global leaders have expressed support , but the British Parliament 's vote against military action earlier this week was a blow to Obama 's hopes of getting strong backing from key NATO allies .

On Saturday , Obama proposed what he said would be a limited military action against Syrian President Bashar al-Assad . Any military attack would not be open-ended or include U.S. ground forces , he said .

Syria 's alleged use of chemical weapons earlier this month `` is an assault on human dignity , '' the president said .

A failure to respond with force , Obama argued , `` could lead to escalating use of chemical weapons or their proliferation to terrorist groups who would do our people harm . In a world with many dangers , this menace must be confronted . ''

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Obama decision came Friday night

On Friday night , the president made a last-minute decision to consult lawmakers .

What will happen if they vote no ?

It 's unclear . A senior administration official told CNN that Obama has the authority to act without Congress -- even if Congress rejects his request for authorization to use force .

Obama on Saturday continued to shore up support for a strike on the al-Assad government .

He spoke by phone with French President Francois Hollande before his Rose Garden speech .

`` The two leaders agreed that the international community must deliver a resolute message to the Assad regime -- and others who would consider using chemical weapons -- that these crimes are unacceptable and those who violate this international norm will be held accountable by the world , '' the White House said .

Meanwhile , as uncertainty loomed over how Congress would weigh in , U.S. military officials said they remained at the ready .

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Reactions mixed to Obama 's speech

A spokesman for the Syrian National Coalition said that the opposition group was disappointed by Obama 's announcement .

`` Our fear now is that the lack of action could embolden the regime and they repeat his attacks in a more serious way , '' said spokesman Louay Safi . `` So we are quite concerned . ''

Some members of Congress applauded Obama 's decision .

House Speaker John Boehner , Majority Leader Eric Cantor , Majority Whip Kevin McCarthy and Conference Chair Cathy McMorris Rodgers issued a statement Saturday praising the president .

`` Under the Constitution , the responsibility to declare war lies with Congress , '' the Republican lawmakers said . `` We are glad the president is seeking authorization for any military action in Syria in response to serious , substantive questions being raised . ''

More than 160 legislators , including 63 of Obama 's fellow Democrats , had signed letters calling for either a vote or at least a `` full debate '' before any U.S. action .

British Prime Minister David Cameron , whose own attempt to get lawmakers in his country to support military action in Syria failed earlier this week , responded to Obama 's speech in a Twitter post Saturday .

`` I understand and support Barack Obama 's position on Syria , '' Cameron said .

An influential lawmaker in Russia -- which has stood by Syria and criticized the United States -- had his own theory .

`` The main reason Obama is turning to the Congress : the military operation did not get enough support either in the world , among allies of the US or in the United States itself , '' Alexei Pushkov , chairman of the international-affairs committee of the Russian State Duma , said in a Twitter post .

In the United States , scattered groups of anti-war protesters around the country took to the streets Saturday .

`` Like many other Americans ... we 're just tired of the United States getting involved and invading and bombing other countries , '' said Robin Rosecrans , who was among hundreds at a Los Angeles demonstration .

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Syria 's government unfazed

After Obama 's speech , a military and political analyst on Syrian state TV said Obama is `` embarrassed '' that Russia opposes military action against Syria , is `` crying for help '' for someone to come to his rescue and is facing two defeats -- on the political and military levels .

Syria 's prime minister appeared unfazed by the saber-rattling .

`` The Syrian Army 's status is on maximum readiness and fingers are on the trigger to confront all challenges , '' Wael Nader al-Halqi said during a meeting with a delegation of Syrian expatriates from Italy , according to a banner on Syria State TV that was broadcast prior to Obama 's address .

An anchor on Syrian state television said Obama `` appeared to be preparing for an aggression on Syria based on repeated lies . ''

A top Syrian diplomat told the state television network that Obama was facing pressure to take military action from Israel , Turkey , some Arabs and right-wing extremists in the United States .

`` I think he has done well by doing what Cameron did in terms of taking the issue to Parliament , '' said Bashar Jaafari , Syria 's ambassador to the United Nations .

Both Obama and Cameron , he said , `` climbed to the top of the tree and do n't know how to get down . ''

The Syrian government has denied that it used chemical weapons in the August 21 attack , saying that jihadists fighting with the rebels used them in an effort to turn global sentiments against it .

British intelligence had put the number of people killed in the attack at more than 350 .

On Saturday , Obama said `` all told , well over 1,000 people were murdered . '' U.S. Secretary of State John Kerry on Friday cited a death toll of 1,429 , more than 400 of them children . No explanation was offered for the discrepancy .

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Syrian official : Obama climbed to the top of the tree , `` does n't know how to get down ''

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Obama sends a letter to the heads of the House and Senate

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Obama to seek congressional approval on military action against Syria

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Aim is to determine whether CW were used , not by whom , says U.N. spokesman